Measurement solution for FTIR Vs. GC in gas analysis - Monitoring and Testing

Comparison of Technologies

A gas chromatograph (GC) has traditionally been the technology of choice for laboratory analysis of gas samples. GCs have been used for over 50 years in the qualification and quantification of complex mixtures of gases. The techniques and methodology required to run GCs are often a pre-requisite for any laboratory technician. Recent developments in Fourier Transform Infra-Red (FTIR) gas analysis has enabled this technology to start to replace GCs in the lab.

Both technologies are ideally suited to the identification and quantification of gas species. We discuss here the advantage and disadvantages of both techniques.

Gas Chromatography is a mass separation method. A carrier gas is permanently blown through a separation column, which is often either heated or temperature programmed. The outlet from the column goes through a detector of which there are different types. The sample to be tested is injected into the front of the column where it is separated into constituent components which appear as peaks at the detector one after the other. For different gas species or detection limits, various detectors are used. The peaks are identified by the retention time in the column after injection, given the conditions of flow and temperature. To calibrate, pure compound samples are injected in the same way and the retention times noted. The concentration is calculated from detector signal by comparing the size of the peak of the sample with the same for a concentration standard.

IR absorption spectroscopy measures the absorption of light by the molecules of the gas, based on the molecular vibrations within the gas species present. The IR absorption is a function of type of vibration within in the molecule, as well as the temperature, pressure, concentration and pathlength (distance the IR light travels in the absorbing species). The absorption is carried out in a closed gas cell, where the IR light is reflected a number of times, giving the specific pathlength. For selective gas applications, the absorption at a single frequency can be measured giving a measurement result for one gas. However with a FTIR spectrometer the full IR spectrum can be collected, enabling the detection of the absorptions peaks of all gases within the sample at once. By using chemometric software and a suitable calibration set of spectra at known concentrations, the full spectrum can be separated into the responses of each gas of interest. Quantification is made by the chemometric algorithm, with the linear absorption response of the sample being modelled against the calibration set.

Specific Advantages of Protea atmosFIR analysers

Protea’s atmosFIR range of FTIR analysers are dedicated gas analysers. As opposed to standard laboratory spectrometers that have gas cells added into the sample compartment, atmosFIR has been designed as the optimum tool for multi-species gas analysis. Software and hardware is combined to give control of the sample and analysis of the gas in real-time.

With dedicated gas analysers based on FTIR technology, such as Protea’s atmosFIR platform, quick, repeatable and cost-effective measurements can now be made using the power of IR spectroscopy.